Construction of an M3L2A6 cage with small windows from a flexible tripodal ligand and Cu(hfac)3

Article abstract of DOI:10.1021/ic402610q

An M₃L₂A₆ cage has been prepared with small windows from a tripodal ligand, L, and Cu(hfac)₂. Cold spray ionization mass spectrometry of a mixture of L and Cu(hfac)₂ revealed the formation of a Cu₃L₂hfac₆ cage in solution. X-ray crystallography showed that the Cu₃L ₂hfac₆ cage included neutral molecules such as THF and CHCl₃. Furthermore, the six hfac anions have been shown to play an important role in holding neutral guest molecules securely in place.

Full text of DOI:10.1021/ic402610q

Communication
pubs.acs.org/IC
Construction of an M3L2A6 Cage with Small Windows from a Flexible
Tripodal Ligand and Cu(hfac)3
Mari Ikeda,† Keiji Ohno,‡ Yuki Kasumi,‡ Shunsuke Kuwahara,†,‡ and Yoichi Habata*,†,‡
†Research Centre for Materials with Integrated Properties and ‡Department of Chemistry, Faculty of Science, Toho University, 2-2-1
Miyama, Funabashi, Chiba 274-8510, Japan
S
* Supporting Information
ABSTRACT: An M3L2A6 cage has been prepared with
small windows from a tripodal ligand, L, and Cu(hfac)2.
Cold spray ionization mass spectrometry of a mixture of L
and Cu(hfac)2 revealed the formation of a Cu3L2hfac6 cage
in solution. X-ray crystallography showed that the
Cu3L2hfac6 cage included neutral molecules such as THF
and CHCl3. Furthermore, the six hfac anions have been
shown to play an important role in holding neutral guest
Figure 1. Side view (a) and top view (b) of a cage structure with small
molecules securely in place.
windows and our new flexible tripodal ligand (c). Red, green, and blue
tubes mean L, Cu2+, and hfac anions, respectively.
he formation of molecular cages by self-assembly has
Recrystallization of the green powder from a mixture of CHCl3
1
Tattracted considerable attention from many researchers,
and hexane (1:1, v/v) gave the [CHCl3⊂Cu3L2hfac6] complex.
and various tripodal ligands have been reported in the literature
The X-ray crystal structure of the [CHCl3⊂Cu3L2hfac6] complex
in this particular context.2 For example, Fujita et al.2u reported
is shown in Figure S3 in the Supporting Information (SI).
that the combination of 1,3,5-tris(4-pyridylmethyl)benzene and
Elemental analysis of the complex indicated that it was 2:3 L/
Cu(hfac)2. Figure 2 shows the crystal structure of the Cu(hfac)2
enPd(NO3)2 led to the formation of a cagelike complex, and
Kaim et al.2v reported that different combinations of AgBF4 or
complex with L. The complex formed an M3L2A6-type cage
containing two THF molecules in its cavity. Each Cu2+ ion is six-
CuI and 1,3,5-tris(benzimidazol-1-ylmethyl)-2,4,6-trimethylben-
−
zene led to the formation of metallocages containing BF4 or
CuI3− anions in their cavity. In these cases, the counteranions of
coordinated by four O atoms of the hfac anions and two N atoms
of the pyridines of L. The four O atoms and the Cu2+ ion lie in the
same plane, and the N−Cu−N angles are in the range 174.7−
178.5°. The mean N(pyridine)−Cu distance was found to be
2.000 Å and therefore comparable to that of a pyridine/
Cu(hfac)2 complex.3k The void volume of the Cu3L2hfac6 cage
was estimated to be 285 Å3.7 Lindoy et al.8 reported that the two
di-β-diketone ligands 1,10-(1,4-phenylene)dihexane-1,3-dione
(L1) and 1,1′-biphenyl-4,4′-diylbis(4,4,4-trifluorobutane-1,3-
dione) (L2) formed large neutral open-faced Fe4L6 tetrahedral
−
−
the metal salts (e.g., NO3 , BF4 , and I−) did not participate in
the formation of cage structures, indicating that these cages
contained large windows through which the anions could readily
pass.
Hexafluoroacetylacetonate (hfac) salts have been used in a
number of studies to form supramolecular systems such as
coordination polymers,3 metallomacrocycles,4 and clusters.5 In
these supramolecular structures, the hfac anions bind tightly to
the metal cations, and Cu(hfac)2 is about 280 Å3.6 On the basis of
these observations, it was envisaged that the combination of hfac
salts with a tripodal ligand would lead to the formation of a new
type of cage with small windows, where the hfac anions would
effectively form a barrier, preventing escape of the guest
molecules and leading to enhanced binding properties (Figure
1a). Herein, we report the combination of a flexible tripodal
ligand and Cu(hfac)2 to construct a new type of three-
dimensional cage structure with small windows.
A new tripodal ligand (Figure 1b, L) was prepared in 45% yield
by the reaction of tris(4-bromobenzeyl)amine (2) with 3-
pyridineboronic acid.
Single crystals of the Cu(hfac)2 complex with L were obtained
as follows. A solution of L in CHCl3 was mixed with a solution of
Cu(hfac)2 in CH3OH to give a green powder, which was
recrystallized from a mixture of tetrahydrofuran (THF)/hexane
(1:1, v/v) to give single crystals of the desired material.
cage complexes. The Fe4L1 cage had a small cavity (void volume
6
= 174 Å3), whereas the Fe4L2 cage had a larger cavity (void
6
volume = 844 Å3). The Fe4L16 and Fe4L26 cages contained one
and four THF molecules, respectively. It is therefore important
to note that the Cu3L2hfac6 cage with a small void volume (285
Å3) and small windows held on tightly to two molecules of THF
(total 163.5 Å3)9 using its ligand framework and the six hfac
anions (Figure 2b). Two atropisomers (Δ and Λ forms) existed
for the conformation of L (Figure 2c), and the
[2THF⊂Cu3L2hfac6] complex existed in its racemic form in
the solid state.
The [2THF⊂Cu3L2hfac6] complex was subjected to thermog-
ravimetric−differential thermal analysis (TG-DTA). As shown in
Figure 3, the weight losses at 93 and 176 °C were 2.7 and 5.5%,
Received: October 16, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ic402610q | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
example indicated that the Cu3L2hfac6 cage was holding onto its
THF molecules tightly.
Cold spray ionization mass spectrometry (CSI-MS) was used
to determine whether L formed a cage structure with Cu(hfac)2
in a solution at 298 K, using a mixture of THF and CH3OH (1:1,
v/v). As shown in Figure 4, a base peak corresponding to [L +
Figure 4. CSI-MS of the Cu3L2hfac6 cage in THF/CH3OH (1:1, v/v) at
298 K. The hfac anions are omitted in the cage models.
Cu2+ + hfac−]+ was observed at m/z 788. Fragment-ion peaks
corresponding to [2L + Cu2+ + hfac−]+ and [2L + 2Cu2+
+
3hfac−]+ were also observed at m/z 1306 and 1785, respectively.
Furthermore, a fragment-ion peak corresponding to [2L + 3Cu2+
+ 5hfac−]+ was observed at m/z 2262. These CSI-MS data
strongly supported the idea that the Cu(hfac)2 complex with L
formed a cage structure in solution. These fragment-ion peaks,
however, did not include THF molecules because the lack of hfac
anions effectively opened the cage windows and allowed the
THF molecules to readily escape from the complex.
Figure 2. Side view (a), top view without the THF molecules (b), and
atropisomers without the THF molecules and hfac anions (c) of the
[2THF⊂Cu3L2hfac6] complex. The hfac anions are colored blue in part
b.
The binding constant between L and Cu(hfac)2 was estimated
by Cu(hfac)2-induced titration experiments using UV−vis in
CH3OH (Figure 5 and the associated multimedia file in the SI).
log K for the 2:3 2/Cu(hfac)2 complex in CH3OH was estimated
to be 18.1 based on nonlinear least-squares curve fitting.11
Walker and Li12 reported that log β2 for a 2:1 4-methylpyridine/
Cu(hfac)2 complex was much less than that of the current
complex at 2.7. The lower log β2 value of the 4-methylpyridine/
Figure 3. TG-DTA of the [2THF⊂Cu3L2hfac6] complex.
respectively. These weight losses were attributed to the
sequential loss of two molecules of THF (0.98 and 1.01
molecules in the first and second steps, respectively). TG-DTA
also indicated that the Cu3L2hfac6 cage retained some of its THF
molecules up to a temperature of 176 °C. Ananchenko et al.10
reported that a capsule composed of two molecules of p-
alkanoylcalix[4]arene encapsulates four molecules of THF in its
cavity. In this case, all of the THF molecules remained within the
inclusion complex until the temperature reached 140 °C. Beyond
this temperature, the THF molecules left the complex. This
Figure 5. Changes in the UV−vis spectra of L upon the addition of
Cu(hfac)2 in CH3OH.
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Inorganic Chemistry
Communication
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stabilized by the synergistic coordination of six pyridines.
In conclusion, we have demonstrated that the new tripodal
ligand, L, forms an M3L2A6 cage with Cu(hfac)2. This is the first
reported example of a cage compound with small windows
composed of hfac anions. TG-DTA indicated that the hfac anions
played an important role in holding the guest molecules tightly.
The application of this system to enhance the circular dichroism
(CD) intensity of chiral molecules with small CD molecular
ellipticity is now underway in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
Crystallographic data in CIF format, 1H and 13C NMR spectra of
compounds, conditions for the titration experiments, X-ray
structure of the [CHCl3⊂Cu3L2hfac6] complex, crystal data, and
UV−vis spectra and speciation analyses, which are also available
as a multimedia file (habata.avi). This material is available free of
charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: habata@chem.sci.toho-u.ac.jp.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Professor Masatoshi Hasegawa for the TG-
DTA measurements. This research was supported by Grants-in-
Aid 08026969 and 11011761, a High-Tech Research Center
project (2005−2009), and the Supported Program for Strategic
Research Foundation at Private Universities (2012−2016) from
the Ministry of Education, Culture, Sports, Science and
Technology of Japan for Y.H.
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dx.doi.org/10.1021/ic402610q | Inorg. Chem. XXXX, XXX, XXX−XXX